This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems with aberration correction based upon harmonic and non-harmonic signal analysis.
The geometric delay calculations used by ultrasonic transducer array beamformers are generally predicated upon the assumption that the speed of sound through the body being imaged is a known constant. However, the reality is that the body generally provides non-homogeneous tissue paths which can cause different phase and amplitude distortions and arrival time fluctuations to the wave fronts across the array transducer aperture. As a result, an exact focus of transmitted and received beams may not always occur. In theory many of these effects can be overcome by adjustment of the delays used to focus the transmitted and received beams. Determining the delay adjustments needed, and doing so adaptively and in real time, have been the subject of investigations for many years. One approach involves cross correlating echoes received by neighboring transducer elements or groups of elements to estimate arrival time differences. Another approach has been to vary focusing delays so as to maximize the brightness of speckle or reflectors in the image field. While many of these efforts have tried to provide compensation for time shifts in the receiving aperture, others have looked to provide time-shift compensation for the transmit aperture. Many of these approaches are iterative in nature, and face the problems of converging to an acceptable result with an infinitely variable and at times moving combination of reflectors and aberrators in the image field. All have faced the challenges of intensive computation and signal processing and the need to know when the appropriate level of compensation has been provided.
U.S. Pat. Nos. 6,023,977, 6,131,458, and 6,223,599 have addressed the problem from the perspective of the analysis of signals of different frequencies, in particular, various combinations of fundamental and harmonic frequency signals. The various embodiments proposed by these patents use fundamental or harmonic or broadband frequencies to compute aberration correction estimates for fundamental or harmonic images. In several embodiments both the fundamental and harmonic bands of the echo signals are used to derive separate aberration correction estimates, which are then averaged together in the hope that one of the estimates is more stable and will dominate the result. Of course if one estimate is correct and the other erroneous, the correction is made less accurate by the averaging process: The algorithms used for aberration estimation in these patents are variations of the cross-correlation technique mentioned above, in which signals from a subarray of four elements are correlated with signals from an adjacent subarray; the signals from a subarray are correlated against a sum of subarray signals; or the signals from a subarray are correlated against a previously stored subarray signal. These algorithms all derive relative phase adjustments rather than adjustments based upon any known or absolute reference. It is desirable to provide an aberration correction technique which is fast, accurate, and provides corrections toward an absolute measure of image quality.
In accordance with the principles of the present invention, aberration corrections are computed by comparing harmonic and non-harmonic images to derive aberration correction estimates. In a preferred embodiment the harmonic image is a reference image against which aberrations in the non-harmonic image are compared. A preferred acquisition technique is to transmit at a frequency f and receive at a frequency n*f to acquire the harmonic image and to transmit at a frequency n*f and receive at a frequency n*f to acquire the non-harmonic image. In a preferred embodiment the aberration correction estimates are produced by back-propagating the image data to find the aperture correction data for the two images. The inventive technique is particularly useful for aberration correction of data from a two dimensional array which is divided into Nxc3x97M subarrays.